1. Field of the Invention
This invention relates generally to inflatable-type modular occupant restraint systems for passenger vehicles or, as they are more commonly known, air bag restraint systems. Such a system may be installed in an automobile or other vehicle, at least in part on the steering wheel or adjacent to the driver for the protection of the driver and also in the dashboard or adjacent to an occupant for passenger protection in the event of a collision. More particularly, this invention relates to an improved means for controlling the discharge rate of gas from an air bag cushion and for preventing the over-pressurization of the air bag cushion.
2. Description of Related Art
Air bag restraint systems for restraining the movement of occupants of a motor vehicle during a collision have long been known. The air bag restraint system is located in close proximity to the occupants and normally is in an uninflated condition. In the event of a crash, however, the air bag is inflated in approximately 20 to 40 milliseconds by a gas produced by the burning of a gas generant material in a gas generator or, as it is more commonly known, an inflator. The gas generant is ignited by an initiator. The pressure of combustion gases resulting from the rapid burning of the gas generant material causes gas to rush into the bag to effect a rapid rate of inflation thereof. The gas generated by the inflator is filtered by a filter unit internal to the inflator.
An air bag cushion, which is normally made from a material that may be substantially impermeable to the flow of gas, typically is provided in part thereof with at least one vent port which serves as an outlet for gas contained within the air bag cushion. Alternatively, the air bag cushion may be provided in part thereof with a permeable fabric which functions as an outlet for gas contained within the cushion. The vent port or permeable fabric for typical operating conditions relieves excess pressure from within the air bag cushion to prevent the cushion from bursting and to prevent recoil of an occupant who impacts onto an overpressurized air bag cushion. When the gas generating material is more reactive due to storage at a high temperature; i.e., storage in a vehicle on a bright sunlit day, when the vehicle is involved in a high-energy collision, or when an extremely large occupant is present the vent means may not afford adequate pressure relief for the air bag cushion. It is preferable to provide a secondary pressure relief route to accommodate the release of increased gas pressure in a controlled manner from the air bag cushion.
Solutions to this problem have been accomplished by using rupturable patches, seams or the like which break and open an exhaust vent to relieve excess pressure from within the air bag cushion. Such solutions are disclosed in Takada, U.S. Pat. No. 4,805,930; Okada et al., U.S. Pat. No. 4,097,065; Nakajima et al., U.S. Pat. No. 5,016,913; and Carey et al., U.S. Pat. No. 3,527,475.
As disclosed by Takada, stitched seams between fabric elements of an air bag cushion induce localized distension when the air bag cushion is exposed to excess pressure. The air bag cushion opens up along the seams causing the film coating of the envelope fabric to rupture along the seams. This allows gas to escape and maintain a substantially constant overall maximum pressure, regardless of variation in ambient temperature.
In Okada, the air bag gas vent is covered with a gas-permeable cloth which is provided with at least one dart.. When the dart tears, it provides a bulge-out portion to relieve pressure within the air bag cushion to prevent the cushion from bursting.
Heat-shrink material which partially blocks a gas vent is disclosed by Nakajima. The shrink-wrap shrinks to increase the area for the passage of gas from the air bag cushion when gas is introduced into the air bag cushion from the inflator.
A patch covering a gas vent is disclosed in Carey. The patch is made from material which has less strength than the material from which the air bag cushion is made. The patch is attached to the cushion with adhesive or similar means. When the air bag cushion is overpressurized, the patch ruptures along the periphery of the gas vent forming a flap which moves to permit gas to exhaust from the air bag cushion.